Bimetallic Pt-Ni dendrimer-encapsulated nanoparticles (DENs) with different Pt/Ni ratios have been successfully synthesized through the co-complexation of Pt2+ and Ni2+ cations to the internal tertiary amine of fourth-generation hydroxyl-terminated poly(amidoamine) dendrimers (G4-OH) followed by coreduction by NaBH4 in aqueous solution. The catalytic activities for the decomposition of hydrous hydrazine and the hydrolysis of ammonia borane over the Pt-Ni DENs have been studied in aqueous solution, and the Pt-Ni DENs show high catalytic performances for these hydrogen-generation reactions. TEM measurements reveal that the size-controlled Pt-Ni NPs, which have an average particle size of 1.1±0.3 nm, are encapsulated in the G4-OH dendrimer.